1. Field of the Invention
2. Description of the Prior Art
Th invention relates to a radial piston pump for generating high fuel pressure in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft, which is rotatably supported in a pump housing and is embodied eccentrically, and having a plurality of pistons, in particular three, each disposed in a respective element bore radially relative to the drive shaft, which are movable radially back and forth in the respective element bore by rotation of the drive shaft and each define one cylinder chamber, which communicates via a suction valve with a low-pressure chamber.
In common rail injection systems, a high-pressure pump, with the aid of a prefeed pump, pumps the fuel to be injected out of a fuel tank into a central high-pressure fuel reservoir, also called a common rail. From the rail, fuel lines lead to the individual injectors, which are assigned to the cylinders of the engine. The injectors are triggered individually by the engine electronics, as a function of the engine operating parameters, in order to inject fuel into the combustion chamber of the engine.
As the high-pressure pump, a radial piston pump can be used, of the kind described for instance in German Patent Application DE 198 48 035, which had not yet been published by the priority date of the present application. In such a radial piston pump, one piston with the associated cylinder chamber each form one pump element, by which the pumped fuel is subjected to high pressure. The pumping quantity of the radial piston pump can be adapted to the demand of the engine at the time with the aid of a metering unit. Then only as much fuel as is actually used is compressed in the radial piston pump. As the metering unit, an adjustable throttle provided on the intake side of the radial piston pump can be employed. As a consequence, in operation with a small pumping quantity, the individual cylinder chambers are incompletely filled. An equivalent of DE 198 48 035 has now issued as U.S. Pat. No. 6,457,957, the disclosure of which is hereby incorporated by reference.
To seal off the individual cylinder chambers from the low-pressure chamber, suction valves prestressed by a closing spring can be used. In practice, different suction valves of the same model have different prestressing, for reasons dictated by their manufacture. Accordingly the opening pressures of the various suction valves also differ. As a consequence, different structurally identical suction valves of the same radial piston pump are opened for different lengths of time, especially at minimal pumping, as a function of the prestressing of the respective closing spring. In an extreme case, it can happen that a suction valve will not open at all. Hence the corresponding cylinder chamber is filled only inadequately if at all in the intake stroke. This results in uneven pumping of the various pump elements. This in turn can cause rough operation of the engine, which is especially unwanted in passenger cars.
In DE 198 48 035, which had not yet been published by the priority date of the present application, it is proposed that uniform pumping of the various pump elements be improved by bracing the closing springs of all the suction valves on the associated pistons. As a result, opening of the suction valves is controlled as a function of the piston position. If one piston is moving toward the associated suction valve, then an increasing force is exerted by the closing spring on the valve body of the suction valve. This assures that the suction valve is closed if the associated piston is at its top dead center. When the piston moves away from the suction valve, the force exerted by the closing spring on the valve body of the suction valve drops. It is thus assured that the suction valve is open when the associated piston is at bottom dead center. However, the embodiment described in DE 198 48 035 has the disadvantage of being relatively complicated and expensive to make.
The primary object of the invention is therefore to furnish a radial piston pump which assures uniform pumping of the various pump elements and which can be produced economically.
In a radial piston pump for generating high fuel pressure in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft, which is rotatably supported in a pump housing and is embodied eccentrically, and having a plurality of pistons, in particular three, each disposed in a respective element bore radially relative to the drive shaft, which are movable radially back and forth in the respective element bore by rotation of the drive shaft and each define one cylinder chamber, which communicates via a suction valve with a low-pressure chamber, this object is attained in that precisely one suction valve is controlled such that it opens and closes as a function of the angle of rotation of the drive shaft.